JP4506985B2 - Extra heavy steel material and method for manufacturing the same - Google Patents

Description

  The present invention relates to an extra heavy steel material and a method for manufacturing the same. Specifically, an ultra-thick steel material suitable as a shape steel or flat steel used in the fields of architecture, civil engineering and offshore structures, and its manufacturing method, in particular, a super extra-thickness H with a flange or web thickness exceeding 80 mm The present invention relates to a very thick steel material suitable as a shape steel and a method for producing the same.

  In recent years, with the increase in the height and size of buildings, there has been a demand for shape steels and flat steels, which are thicker than before and excellent in performance, especially H-section steels.

  Specifically, the thickness is 40 to 125 mm, the yield strength is 415 MPa or more, the tensile strength is 550 MPa or more, and the Charpy absorbed energy at 0 ° C. using a V-notch test piece is 47 J or more. There is a demand for an H-section steel (hereinafter also referred to as “toughness”). There is also a demand for an H-section steel having extremely good impact characteristics such that the absorbed energy is 100 J or more or 200 J or more.

  In response to such demands, Patent Document 1 discloses that strength and toughness that can ensure high strength, high toughness, and high weldability without generating residual stress and strain are small in the variation of strength and toughness in the flange plate thickness direction. As a method for producing an extremely thick H-section steel with excellent weldability, a “method for producing an ultra-thick H-section steel” is disclosed in which a steel material having a specific chemical composition is heated, rolled and cooled under specific conditions. Has been.

  In addition, in Patent Document 2, as a method for producing an extremely thick H-section steel having a flange thickness with a small material difference at each flange portion exceeding 40 mm, heating and rolling under a specific condition for steel having a specific chemical composition And “a method for producing an ultra-thick H-section steel” for cooling.

  Furthermore, in Patent Document 3, as a method for producing an extremely thick H-section steel having a small material difference in each part of the flange and an excellent cross-sectional shape, rolling and cooling under specific conditions are performed on steel having a specific chemical composition. A “rolled shape steel manufacturing method” to be applied is disclosed.

  Patent Document 4 discloses a steel for building structures represented by H-section steel, which has a large plastic deformation capability and can obtain high joint performance even when short bead welding is performed or when the temperature between passes is high, and As a manufacturing method thereof, “steel for building structure and manufacturing method thereof” is disclosed in which a steel material having a specific chemical composition is heated, rolled and cooled under specific conditions.

Japanese Patent Laid-Open No. 10-68016 JP 2001-262225 A JP 2001-286901 A JP 2002-294391 A

The technique proposed in the above-mentioned Patent Document 1 needs to use a relatively high Ar ₃ point of 740 to 775 ° C. for the steel material. For this reason, in the case of a very thick steel material having a slow cooling rate after the end of rolling, especially a very thick steel material having a plate thickness exceeding 80 mm, a coarse ferrite structure is likely to occur, and good mechanical properties are always obtained. It wasn't.

  In the technique proposed in Patent Document 2, Ti is added for the purpose of suppressing the coarsening of the weld heat affected zone (hereinafter referred to as “HAZ”) by TiN and improving the HAZ toughness. B may be added for the purpose of increasing the strength and improving HAZ toughness by bainite, but no consideration has been given to optimizing the Ti and B contents relative to the N content. . For this reason, an extremely thick base material, in particular, an extremely thick base material having a thickness exceeding 80 mm, cannot always ensure good mechanical properties.

  In the technique proposed in Patent Document 3, no consideration is given to optimizing the Ti and B contents with respect to the N content. For this reason, in the case of an extremely thick steel material, especially an extremely thick steel material having a plate thickness exceeding 80 mm, it is not always possible to ensure good mechanical properties in the base material and the welded portion.

  In the case of the technique proposed in Patent Document 4, the steel material contains B in order to fix the free N and to utilize the function of improving the hardenability by free B (solid solution B). Yes. However, the content of Ti, N and B and the thermal history of the steel material have a great influence on the amount of free B. For this reason, in the case of an extra-thick steel material, especially an extra-thick steel material having a plate thickness exceeding 80 mm, good hardenability cannot always be ensured, and mechanical properties may vary.

  Therefore, an object of the present invention is to have a tensile strength characteristic of a yield point of 415 MPa or more and a tensile strength of 550 MPa or more and an impact characteristic that Charpy absorbed energy at 0 ° C. using a V-notch test piece is 47 J or more. It is to provide an extra-thick steel material suitable for steel or flat steel and a method for producing the same, particularly an ultra-thick H-shaped steel having a thickness exceeding 80 mm and a method for producing the same.

  The present inventor has made various studies in order to obtain an ultra-thick steel material having the above-described mechanical properties. As a result, the following findings (a) to (d) were obtained.

  (A) Ultra-thick steel materials, and especially desired mechanical properties for ultra-thick steel materials with a plate thickness exceeding 80 mm, that is, tensile strength characteristics such as a yield point of 415 MPa or more and a tensile strength of 550 MPa or more, and a V-notch test Content of C, Si, Mn, Cr, V, Nb, Ti, B, Al, and N in order to stably provide an impact property with a Charpy absorption energy at 47 ° C. using a piece of 47 J or more It is necessary to strictly control the contents of P, S, Cu, Ni, Mo, and O (oxygen) as impurities.

(B) In order to provide the desired tensile strength characteristics, in addition to the control of the content of the element described above, the element symbol is the content in steel in mass% of the element, and the following formula (1) It is also necessary to control so as to satisfy the value of Ar3 represented by the following formula and the value of f (B) represented by the following formula (2) or (3).
Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1),
“When Ti / N ≦ 3.4”: f (B) = B−0.785 {N− (Ti / 3.4)} (2),
“When Ti / N> 3.4”: f (B) = B (3).

  (C) In order to provide the desired tensile strength characteristics, in addition to the control of the element content and the values of Ar3 and f (B) already described, the proportion of bainite in the structure is 60 to 100%. It is also necessary to do.

  (D) For the steel ingot or steel slab in which the content of the element described in (a) and the values of Ar3 and f (B) described in (b) are controlled, the heating temperature, the rolling end temperature, and the cooling start By applying TMCP technology (thermal processing control technology) with specific conditions of temperature, cooling stop temperature and cooling rate, it is possible to achieve the desired mechanical properties for extra-thick steel plates, especially extra-thick steel materials with a thickness exceeding 80 mm. Certain tensile strength characteristics and impact characteristics can be provided stably.

  The “TMCP technology” is a technology that utilizes reduction in a low temperature range or online cooling, and not only the prescribed mechanical properties but also excellent weldability with a smaller amount of alloy elements than usual. Is a technology that can be obtained.

  The present invention has been completed on the basis of the above findings, and the gist of the present invention resides in the ultra-thick steel material shown in the following (1) and the manufacturing method of the ultra-thick steel material shown in (2).

(1) By mass%, C: 0.041 to 0.09%, Si: 0.11 to 0.6%, Mn: 1 to 1.6%, P: 0.02% or less, S: 0.00. 015% or less, Cu: 0.19% or less, Ni: 0.04% or less, Cr: 0.06-0.9%, Mo: 0.04% or less, V: 0.01-0.1%, Nb: 0.031-0.07%, Ti: 0.003-0.03%, B: 0.0003-0.0029%, Al: 0.003-0.06%, N: 0.0035 0.009% and O (oxygen): 0.006% or less, with the balance being Fe and impurities, the value of Ar3 represented by the following formula (1) is 739 or less, and the following (2) The value of f (B) represented by the formula or the formula (3) has a chemical composition satisfying 0.0004 or less, and the proportion of bainite in the structure is 60 to 10 Thick steel which is a%.
Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1),
“When Ti / N ≦ 3.4”: f (B) = B−0.785 {N− (Ti / 3.4)} (2),
“When Ti / N> 3.4”: f (B) = B (3).
In addition, the element symbol in (1)-(3) Formula represents content in steel in the mass% of the element.

(2) By mass%, C: 0.041 to 0.09%, Si: 0.11 to 0.6%, Mn: 1 to 1.6%, P: 0.02% or less, S: 0.0. 015% or less, Cu: 0.19% or less, Ni: 0.04% or less, Cr: 0.06-0.9%, Mo: 0.04% or less, V: 0.01-0.1%, Nb: 0.031-0.07%, Ti: 0.003-0.03%, B: 0.0003-0.0029%, Al: 0.003-0.06%, N: 0.0035 0.009% and O (oxygen): 0.006% or less, with the balance being Fe and impurities, the value of Ar3 represented by the following formula (1) is 739 or less, and the following (2) A steel ingot or steel slab having a chemical composition satisfying a value of f (B) represented by the formula or formula (3) of 0.0004 or less is a temperature in a temperature range of 1000 to 1350 ° C After heating and hot rolling so that the rolling end temperature is in the temperature range of 980 to 700 ° C., the cooling start temperature is set to 980 to 700 ° C., and the cooling rate is 1 to 10 ° C./sec. A method for producing an extra heavy steel material, characterized by cooling to a temperature in a temperature range of 200 ° C.
Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1),
“When Ti / N ≦ 3.4”: f (B) = B−0.785 {N− (Ti / 3.4)} (2),
“When Ti / N> 3.4”: f (B) = B (3).
In addition, the element symbol in (1)-(3) Formula represents content in steel in the mass% of the element.

  The temperature and the cooling rate in the invention relating to the method for producing an ultra-thick steel material of (2) above indicate values measured at the following sites.

・ Heating temperature of steel ingot or billet: surface of steel ingot or billet
-Rolling end temperature: surface of extra heavy steel,
・ Cooling start temperature: 1/4 thickness from the surface of extra heavy steel
・ Cooling rate: part of thickness 1/4 from the surface of extra heavy steel,
-Cooling stop temperature: A portion of 1/4 thickness from the surface of extra heavy steel.

  Hereinafter, the invention related to the ultra-thick steel material of (1) and the invention related to the manufacturing method of the ultra-thick steel material of (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The ultra-thick steel material of the present invention stably has a tensile strength characteristic of a yield point of 415 MPa or more and a tensile strength of 550 MPa or more and an impact characteristic of Charpy absorbed energy at 0 ° C. using a V-notch test piece of 47 J or more. Therefore, it should be used as a shape steel and flat steel used for various large structures such as high-rise buildings and offshore structures, especially ultra-thick H-shaped steel with a flange or web thickness exceeding 80 mm. Can do. This extra-thick steel material can be obtained relatively easily by the production method of the present invention.

  In addition, since the manufacturing method of the present invention uses rolling at a relatively high temperature and a relatively slow cooling rate, the manufacturing of various heavy thick steels such as H-shaped steel, T-shaped steel and angle steel, and heavy thick flat steel. Suitable for

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition C: 0.041 to 0.09%
C has the effect | action which raises the intensity | strength of a base material and a welding part. However, if the content is less than 0.041%, not only the effect of addition is poor, but it is difficult to ensure the characteristics of the weld metal due to dilution of the base material during welding. On the other hand, if the C content increases, and particularly if the C content exceeds 0.09%, the toughness of the base material and the welded portion is lowered, and weld cracks are likely to occur. Therefore, the content of C is set to 0.041 to 0.09%. The C content is preferably 0.05 to 0.07%.

Si: 0.11 to 0.6%
Si has the effect | action which ensures the intensity | strength of a base material and a welding part. However, if the content is less than 0.11%, the effect of addition is poor. On the other hand, when the Si content increases, particularly when the Si content exceeds 0.6%, the occurrence of weld cracking increases, and the weld zone toughness decreases, particularly the HAZ toughness decreases. Therefore, the Si content is set to 0.11 to 0.6%. The Si content is preferably 0.15 to 0.4%.

Mn: 1 to 1.6%
Mn is an indispensable element for ensuring the strength and toughness of the base material and the weld. However, if the Mn content is less than 1%, a sufficient addition effect cannot be obtained. On the other hand, if the Mn content increases, especially if the Mn content exceeds 1.6%, the hardenability becomes too high and the weldability decreases, and also the weld zone toughness decreases, especially the HAZ toughness. Will cause a decline. Therefore, the Mn content is set to 1 to 1.6%. In addition, it is preferable that content of Mn shall be 1.3 to 1.5%.

P: 0.02% or less P is an element that is unavoidably present in steel as an impurity, segregates at the grain boundary to reduce toughness, and further causes hot cracking during welding. In particular, if its content exceeds 0.02%, the toughness is lowered and the occurrence of hot cracks during welding becomes significant. Therefore, the content of P is set to 0.02% or less. In addition, since P is a more preferable impurity as there are few P, the minimum is not prescribed | regulated in particular.

S: not more than 0.015% S too much promotes center segregation and causes a large amount of stretched MnS, so the mechanical properties of the base material and the welded part, especially the mechanical properties of HAZ Cause deterioration. In particular, when the content exceeds 0.015%, the mechanical properties of the base material and the welded portion are significantly deteriorated. Therefore, the content of S is set to 0.015% or less. In addition, since it is a preferable impurity, so that there is little S, the minimum is not prescribed | regulated in particular.

Cu: 0.19% or less If the content of Cu is 0.19% or less, since it hardly affects the occurrence of surface cracks during hot working, it is acceptable as an impurity. Therefore, the Cu content is set to 0.19% or less. The Cu content is more preferably 0.1% or less.

Ni: 0.04% or less If the content of Ni is 0.04% or less, Ni has almost no effect on the generation of surface flaws during casting of steel ingots, especially surface flaws during continuous casting. Not acceptable as an impurity. Therefore, the Ni content is set to 0.04% or less. The smaller the Ni content, the better.

Cr: 0.06-0.9%
Cr has the effect | action which improves hardenability. In order to obtain this effect with certainty, the Cr content needs to be 0.06% or more. However, when its content exceeds 0.9%, the toughness of the base metal and the welded portion, particularly the HAZ toughness, is reduced. Therefore, the Cr content is set to 0.06 to 0.9%. In addition, it is preferable that content of Cr shall be 0.2 to 0.5%.

Mo: 0.04% or less If Mo content is 0.04% or less, Mo is acceptable as an impurity because it hardly degrades weldability. Therefore, the Mo content is set to 0.04% or less. In addition, it is so preferable that there is little content of Mo.

V: 0.01 to 0.1%
V has an effect of increasing strength. However, if the content is less than 0.01%, sufficient reinforcing action cannot be obtained. On the other hand, if the V content increases, especially when the V content exceeds 0.1%, the toughness and weldability may be lowered. Therefore, the content of V is set to 0.01 to 0.1%. Note that the V content is preferably 0.05 to 0.09%.

Nb: 0.031 to 0.07%
Nb has an effect of improving strength and toughness. However, if the content is less than 0.031%, the above effect cannot be obtained. On the other hand, if the Nb content exceeds 0.07%, not only the effect of improving the strength and toughness of the base metal is saturated, but also the toughness of the welded portion, particularly the HAZ toughness, is significantly reduced. Therefore, the Nb content is set to 0.031 to 0.07%. The Nb content is preferably 0.04 to 0.05%.

Ti: 0.003 to 0.03%
Ti has the effect of improving the surface properties of steel ingots, especially slabs. Ti also has the effect of increasing the toughness of the base material and the weld. In order to obtain these effects, the content needs to be 0.003% or more. However, if the Ti content exceeds 0.03%, the toughness of the base metal is lowered, and the weld toughness, particularly the HAZ toughness, may be lowered. Therefore, the content of Ti is set to 0.003 to 0.03%. The Ti content is preferably 0.007 to 0.02%.

B: 0.0003 to 0.0029%
B has the effect | action which improves the toughness of a base material and a welding part. In order to acquire this effect, the content needs to be 0.0003% or more. However, if the B content exceeds 0.0029%, the toughness of the base material may be reduced, or the welded portion toughness may be reduced. Therefore, the content of B is set to 0.0003 to 0.0029%. The B content is preferably 0.0005 to 0.002%.

Al: 0.003 to 0.06%
Al is an element effective for deoxidation during steelmaking. Al also has the effect of refining crystal grains. The above effect is obtained when the Al content is 0.003% or more. However, when the Al content increases, particularly when the Al content exceeds 0.06%, the amount of inclusions increases and the toughness decreases. Therefore, the content of Al is set to 0.003 to 0.06%. The Al content is preferably 0.006 to 0.04%.

N: 0.0035 to 0.009%
N forms TiN and BN, and when these nitrides are fine, it suppresses the coarsening of austenite grains during high-temperature heating and contributes to increasing toughness by promoting phase transformation. To do. In order to obtain this effect, the content needs to be 0.0035% or more. However, if the N content exceeds 0.009%, the toughness of the base metal may be reduced, or the weld toughness may be reduced. Therefore, the N content is set to 0.0035 to 0.009%. The N content is preferably 0.004 to 0.007%.

O: 0.006% or less O (oxygen) is an impurity inevitably contained in steel. When the content of O increases, particularly when the content of O exceeds 0.006%, the toughness and ductility of the base material and the welded portion are reduced. Therefore, the content of O is set to 0.006% or less. In addition, since O is a more preferable impurity as there are few, the minimum is not prescribed | regulated.

Ar3: 739 or less The value of Ar3 represented by the above formula (1), that is, the formula of “Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb” is the phase from austenite to ferrite during cooling. A guideline for the Ar ₃ point (° C.), which is the temperature at which transformation starts, is shown. If the value of Ar3 exceeds 739, ferrite tends to be formed, and the desired tensile strength characteristics may not be obtained. Therefore, the value of Ar3 is set to 739 or less. In addition, since the toughness may deteriorate when the value of Ar3 is less than 600, the lower limit of the value of Ar3 is preferably 600. The value of Ar3 is more preferably 650 to 730.

f (B): 0.0004 or less In the case of “Ti / N ≦ 3.4”, the above equation (2), that is, “f (B) = B−0.785 {N− (Ti / 3. The value of f (B) represented by “4)}” relates to the amount of free B that is not fixed by N (nitrogen) in the steel. BN precipitates present before heating for hot rolling are dissolved during heating and re-deposited during or after rolling. The amount of free B is not determined only by the element content because the temperature history affects the amount of reprecipitation, but if the value of f (B) is sufficiently small (a negative value may be used), the amount of free B can be reduced. Can be suppressed. In the case of “Ti / N> 3.4”, most of N is fixed by TiN, which is a precipitate more stable than BN. Therefore, the value of f (B) is the above (3). It is expressed by an equation, that is, “f (B) = B”. And when the value of f (B) exceeds 0.0004, toughness may deteriorate. Therefore, the value of f (B) is set to 0.0004 or less. In addition, in the case of “Ti / N ≦ 3.4”, that is, when the value of f (B) is expressed by the above formula (2), the toughness even if the value of f (B) becomes too small. In this case, the lower limit of f (B) is preferably −0.003, and more preferably −0.001 to 0.004.

  For the reasons described above, the ultra-thick steel material according to the present invention (1) contains elements from C to O in the above-mentioned range, the balance is made of Fe and impurities, and Ar3 represented by the above formula (1) It was defined that the chemical composition had a value satisfying a value of 739 or less and f (B) represented by the formula (2) or (3) satisfying 0.0004 or less.

  In addition, in the HAZ of the base material and the welded portion, in order to ensure the effect of the fine nitrides of B and Ti described in the section of N above, “Ti” is the ratio of the Ti content and the N content. / N "is preferably 3 or less.

  Examples of specific chemical components suitable for the ultra-thick steel material according to the present invention include steels a1 to a35 described in Tables 1 and 2. In addition, “0.00” of the Mo content in Cu in steel a9, Ni in steel a10, and steel a12, steel a16, steel a19, steel a20, steel a24, and steel a32 are all less than 0.01. "Means.

(B) Microstructure By making bainite the main structure, the desired mechanical properties of the extra-thick steel material according to the present invention, that is, the tensile strength characteristic of yield point of 415 MPa or more, tensile strength of 550 MPa or more, The impact characteristic that Charpy absorbed energy at 0 ° C. using a V-notch test piece is 47 J or more can be stably provided. The proportion of bainite in the structure may be 100%, in other words, it may be a bainite single-phase structure. However, when the proportion of bainite in the structure is small, particularly when it is less than 60%, it is difficult to ensure the extra-thick steel material with both desired tensile strength characteristics and impact characteristics.

  For the above reasons, the extra-thick steel material according to the present invention (1) is defined such that the proportion of bainite in the structure is 60 to 100%.

  The structure other than bainite in the structure may be any structure (phase) such as martensite, ferrite, or pearlite. However, in order to make the extra heavy steel material have good impact characteristics, it is desirable that the ratio of martensite in the structure is less than 10%.

  It is known that the volume proportion of a phase in the tissue is equal to the area proportion. For this reason, what is necessary is just to use the area ratio measured by normal microstructure observation means, such as an optical microscope, for the ratio of the bainite to said structure | tissue.

  The extra heavy steel material according to the present invention (1) can be manufactured, for example, by the manufacturing method according to the present invention (2).

(C) Manufacturing conditions For the steel ingot or steel slab having the chemical composition described in the above section (A), the heating temperature, the rolling end temperature, the cooling start temperature, the cooling stop temperature, and the cooling rate were specified conditions. By applying the TMCP technology, it is possible to stably provide desired mechanical properties to an extra-thick steel material, in particular, an extra-thick steel material having a plate thickness exceeding 80 mm. That is, an extra-thick steel having a tensile strength characteristic of a yield point of 415 MPa or more and a tensile strength of 550 MPa or more, and further an impact characteristic of Charpy absorbed energy at 0 ° C. of 47 J or more using a V-notch test piece. Can be obtained.

  This will be described below. In addition, as already stated, said temperature and cooling rate point out the value measured in the following site | part, respectively.

・ Heating temperature of steel ingot or billet: surface of steel ingot or billet
-Rolling end temperature: surface of extra heavy steel,
・ Cooling start temperature: 1/4 thickness from the surface of extra heavy steel
・ Cooling rate: part of thickness 1/4 from the surface of extra heavy steel,
-Cooling stop temperature: A portion of 1/4 thickness from the surface of extra heavy steel.

(C-1) Heating temperature The steel ingot or steel slab having the chemical composition described in the above section (A) is preferably heated to a temperature range of 1000 to 1350 ° C. during hot rolling. By setting the heating temperature to 1000 ° C. or higher, Nb, V, and the like are dissolved in the base, so that the strength of the extra-thick steel material that is the final product (target product) can be increased. Moreover, since the coarsening of a crystal grain is prevented by making heating temperature 1350 degreeC or less, a favorable impact characteristic is ensured.

(C-2) Hot rolling end temperature After heating as described in the above section (C-1), it is preferable to perform hot rolling to a predetermined shape and size, and then control cooling. In addition, it is good to perform hot rolling so that rolling completion temperature may become the temperature of a temperature range of 980-700 degreeC. When the rolling end temperature is higher than 980 ° C., it may be difficult to ensure desired impact characteristics. On the other hand, when the rolling end temperature is lower than 700 ° C., it may be difficult to ensure desired tensile strength characteristics.

(C-3) Cooling start temperature After finishing the hot rolling under the conditions described in the items (C-1) and (C-2), the cooling is controlled at a cooling start temperature of 980 to 700 ° C. Is good. When the cooling start temperature is higher than 980 ° C., it may be difficult to ensure desired impact characteristics. On the other hand, when the cooling start temperature is lower than 700 ° C., it may be difficult to ensure desired tensile strength characteristics.

(C-4) Cooling rate The cooling rate for controlled cooling is preferably 1 to 10 ° C / second. Even if the cooling is started from the temperature described in the above section (C-3), if the cooling rate exceeds 10 ° C./second, the variation in mechanical properties depending on the part may increase, When the cooling rate is lower than 1 ° C./second, it may be difficult to ensure desired impact characteristics.

  The cooling rate at the time of controlled cooling is more preferably 1 to 5 ° C./second.

(C-5) Control cooling stop temperature The above control cooling is preferably performed up to a temperature in the temperature range of 650 to 200 ° C. Even if hot-rolling is performed under the conditions described in the items (C-1) and (C-2), and then controlled cooling is performed under the conditions described in the items (C-3) and (C-4). When the cooling stop temperature at the cooling rate is higher than 650 ° C., it may be difficult to ensure desired tensile strength characteristics. In addition, when the cooling stop temperature is lower than 200 ° C., it may be difficult to ensure desired impact characteristics, or hydrogen cracking may easily occur. Note that the controlled cooling at the cooling rate is more preferably performed to a temperature in the temperature range of 600 to 400 ° C.

  By rolling and controlled cooling under the conditions described in the above sections (C-1) to (C-5), the desired mechanical properties of the extra heavy steel material, that is, a yield point of 415 MPa or more, A tensile strength characteristic of a tensile strength of 550 MPa or more and an impact characteristic of a Charpy absorbed energy at 0 ° C. using a V-notch test piece of 47 J or more are stably secured.

  For this reason, the manufacturing method of the ultra-thick steel material according to the present invention (2) heats the steel ingot or steel slab having the chemical composition described in the section (A) to a temperature in the temperature range of 1000 to 1350 ° C. After hot rolling so that the rolling end temperature is in the temperature range of 980 to 700 ° C., the cooling start temperature is 980 to 700 ° C., and the cooling rate is 1 to 10 ° C./second, and the temperature is 650 to 200 ° C. It was decided to cool to the temperature of the zone.

  The cooling conditions after the controlled cooling is stopped at the temperature described in the above section (C-5) are not particularly stipulated. Therefore, the temperature is lowered to room temperature (room temperature) by appropriate means such as cooling in the air. It only has to be cooled.

  As already described, the extra heavy steel material according to the present invention (1) can be obtained relatively easily by the method of the present invention (2).

  That is, the steel having the chemical composition described in the above section (A), that is, the steel having the chemical composition defined in the present invention, for example, is melted in a converter and cast into a slab by a continuous casting method. Slab heating, rough rolling using perforated rolling, edger rolling under the conditions described in the above items (C-1) to (C-5), that is, the conditions specified in the present invention (2) By performing hot rolling consisting of intermediate rolling using a rolling mill and a rough universal rolling mill and finishing rolling using a finishing universal rolling mill, and performing controlled cooling after hot rolling, the target mechanical properties, that is, Flanges or wedges satisfying the tensile strength characteristics of yield point of 415 MPa or more and tensile strength of 550 MPa or more and the impact characteristics of Charpy absorbed energy at 0 ° C. using a V-notch test piece of 47 J or more. The thickness of the probe it is possible to produce ChokyokuAtsu H-beams exceeding 80 mm.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. For example, it is also possible to manufacture a steel material with a thin plate thickness, and they are all included in the technical scope of the present invention.

  Steel 1 and steel 2 having the chemical composition shown in Table 3 were melted using a 180 kg vacuum melting furnace and cast into a mold to form a steel ingot. Steel 1 is a steel according to the present invention whose chemical composition is within the range specified in the present invention (1), and Steel 2 is a steel according to a comparative example whose components are out of the content range defined in the present invention (1). . In Table 3, “0.00” in the contents of Cu, Ni, and Mo all mean “less than 0.01”.

  Each steel ingot was heated to 1250 ° C. and then hot forged to produce three steel pieces each having a thickness of 200 mm, a width of 160 mm, and a length of 180 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  A thermocouple was embedded in the thickness direction 1/4, width direction 1/2, and length direction 1/2 of the steel piece having a thickness of 200 mm, a width of 160 mm, and a length of 180 mm obtained in this manner at 1250 ° C. After heating, the steel sheet was hot-rolled according to the pass schedule shown in Table 4 to obtain three types of steel sheets having sheet thicknesses of 89 mm (rolling condition 1), 106 mm (rolling condition 2), and 125 mm (rolling condition 3).

  The rolling in the first pass from the thickness of 200 mm to 185 mm in Table 4 was carried out immediately after taking out the steel piece from the heating furnace in any case of rolling conditions 1 to 3.

  The numbers in the “temperature (° C.)” column in each rolling condition in Table 4 indicate the temperature at the end of rolling in the pass. Rolling end temperature to the final plate thickness under each rolling condition, that is, rolling end temperature at the 15th pass to 89 mm under rolling condition 1, rolling end temperature at the 11th pass to 106 mm under rolling condition 2, In rolling condition 3, each pass No. was adjusted while adjusting the temperature so that the rolling end temperature in the seventh pass to 125 mm was 850 ° C. Rolling in was carried out.

  In addition, all the temperature at the time of completion | finish of rolling in each pass points out the temperature in the steel plate surface.

  After the hot rolling under the conditions shown in Table 4 was finished, the steel 1 was subjected to controlled cooling simulating flange cooling at the time of manufacturing the H-section steel. Control cooling was performed by oil cooling, and after stopping the control cooling, it was taken out into the atmosphere and allowed to cool to room temperature (room temperature).

  On the other hand, after finishing the above hot rolling, the steel 2 was allowed to cool to room temperature (room temperature) as it was in the atmosphere.

  In addition, after finishing hot rolling, while measuring temperature with the thermocouple embedded in the site | part of the thickness of the above-mentioned steel slab, the average cooling rate during control cooling was investigated.

  Table 5 shows the result of temperature measurement with a thermocouple and the average cooling rate determined using the result.

  Each steel plate thus obtained was examined for tensile strength characteristics and impact characteristics as microstructures and mechanical properties.

  The microstructure is obtained by taking a test piece from a position where the thickness is 1/4 and 1/2 from the surface in the center in the width direction of the steel plate, mirror-polishing the surface including the rolling direction and the plate thickness direction, Corrosion was observed, and optical microscope observation was performed at magnifications of 100 and 500, and the proportion of bainite in the structure was investigated.

  The tensile test was performed at room temperature using a round bar tensile test piece having a parallel part diameter of 8.5 mm and a parallel part length of 50 mm, and yield point (YP, where 0.2% proof stress was used). The tensile strength (TS) was measured and the yield ratio (YR) was determined. In addition, said tensile test piece was extract | collected in parallel with the rolling direction (namely, the length direction of a steel plate) from the site | part used as thickness 1/4 and 1/2 in the center part of the width direction of a steel plate. The target of the tensile strength characteristic was to have a yield point of 415 MPa or more and a tensile strength of 550 MPa or more.

  The impact characteristics were obtained by collecting a V-notch test piece described in JIS Z 2242 (2005) from a part having a thickness of 1/4 and 1/2 from the surface of the steel plate in parallel with the rolling direction, and performing a Charpy impact test. Absorbed energy (“vE0”) and ductile-brittle fracture surface transition temperature (“vTs”) at 0 ° C. were measured. The target of impact characteristics was to have a Charpy absorbed energy of 47 J or more at 0 ° C.

  Table 6 shows the results of the above tests.

  From Table 6, although the steel composition of Test Nos. 1 to 3 of “Invention Example” satisfying the conditions defined by the present invention in terms of the chemical composition and the ratio of bainite in the structure is an extremely thick steel material having a thickness exceeding 80 mm. First, the above-mentioned target mechanical properties, that is, a tensile strength characteristic of a yield point of 415 MPa or more and a tensile strength of 550 MPa or more, and an impact characteristic that Charpy absorbed energy at 0 ° C. using a V-notch test piece is 47 J or more. It is clear that

  On the other hand, the steel sheets of test numbers 4 to 6 of “Comparative Example” are inferior to at least one characteristic of yield point, tensile strength, and Charpy absorbed energy at 0 ° C.

The ultra-thick steel material of the present invention stably has a tensile strength characteristic of a yield point of 415 MPa or more and a tensile strength of 550 MPa or more and an impact characteristic of Charpy absorbed energy at 0 ° C. using a V-notch test piece of 47 J or more. Therefore, it should be used as a shape steel and flat steel used for various large structures such as high-rise buildings and offshore structures, especially ultra-thick H-shaped steel with a flange or web thickness exceeding 80 mm. Can do. This extra-thick steel material can be obtained relatively easily by the production method of the present invention.

Claims (2)

In mass%, C: 0.041 to 0.09%, Si: 0.11 to 0.6%, Mn: 1 to 1.6%, P: 0.02% or less, S: 0.015% or less Cu: 0.19% or less, Ni: 0.04% or less, Cr: 0.06 to 0.9%, Mo: 0.04% or less, V: 0.01 to 0.1%, Nb: 0 0.03 to 0.07%, Ti: 0.003 to 0.03%, B: 0.0003 to 0.0029%, Al: 0.003 to 0.06%, N: 0.0035 to 0.009 % And O (oxygen): 0.006% or less, the balance is Fe and impurities, the value of Ar3 represented by the following formula (1) is 739 or less, and the following formula (2) or ( 3) It has a chemical composition satisfying a value of f (B) represented by the formula of 0.0004 or less, and the proportion of bainite in the structure is 60 to 100%. Extra-thick steel characterized by Rukoto.
Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1)
“When Ti / N ≦ 3.4”: f (B) = B−0.785 {N− (Ti / 3.4)} (2)
“When Ti / N> 3.4”: f (B) = B (3)
In addition, the element symbol in (1)-(3) Formula represents content in steel in the mass% of the element. In mass%, C: 0.041 to 0.09%, Si: 0.11 to 0.6%, Mn: 1 to 1.6%, P: 0.02% or less, S: 0.015% or less Cu: 0.19% or less, Ni: 0.04% or less, Cr: 0.06 to 0.9%, Mo: 0.04% or less, V: 0.01 to 0.1%, Nb: 0 0.03 to 0.07%, Ti: 0.003 to 0.03%, B: 0.0003 to 0.0029%, Al: 0.003 to 0.06%, N: 0.0035 to 0.009 % And O (oxygen): 0.006% or less, the balance is Fe and impurities, the value of Ar3 represented by the following formula (1) is 739 or less, and the following formula (2) or ( 3) A steel ingot or steel slab having a chemical composition satisfying a value of f (B) represented by the formula of 0.0004 or less is heated to a temperature in the temperature range of 1000 to 1350 ° C. Then, after hot rolling so that the rolling end temperature becomes a temperature in the temperature range of 980 to 700 ° C., the cooling start temperature is set to 980 to 700 ° C., and the cooling rate is 1 to 10 ° C./second, and 650 to 200 ° C. A method for producing an extra-thick steel material, characterized by cooling to a temperature in the temperature range.
Ar3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1)
“When Ti / N ≦ 3.4”: f (B) = B−0.785 {N− (Ti / 3.4)} (2)
“When Ti / N> 3.4”: f (B) = B (3)
In addition, the element symbol in (1)-(3) Formula represents content in steel in the mass% of the element.